This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. To better understand the vasculopathy of sickle cell disease, we are interested in measuring EPC number and function in people with sickle cell disease and determining whether chronic erythrocyte transfusion therapy corrects abnormalities in number or function. We hypothesize that the number of circulating EPCs are increased in individuals with sickle cell disease relative to normal controls due to increased levels of hematopoietic growth factors and ongoing vessel injury due to vaso-occlusive events. While numbers of EPCs in untransfused sickle cell subjects are likely to be increased, we postulate that they will have impaired vascular tube formation and increased oxidant production. Transfusion therapy, by lowering erythropoietin production, will lower the numbers of circulating EPCs in sickle cell subjects and may restore normal function to EPCs. EPC biology in sickle cell vasculopathy has never been studied. In this pilot project, we propose to investigate whether people with sickle cell disease have abnormal EPC number or function. Elucidation of these relationships will improve our understanding of the etiology of some of the disabling complications of this disease and enhance our ability to develop targeted therapeutic interventions for treatment and prevention of sickle cell vascular complications. HYPOTHESES 1. Individuals with sickle cell disease will have increased numbers of circulating EPCs and outgrowth cells than normal controls, in association with elevated levels of erythropoietin. 2. EPCs from subjects with Hb SS will have reduced vascular tube formation than those from normal controls. 3. EPCs from subjects with Hb SS will have a pro-oxidant phenotype compared to normal controls. 4. Subjects with sickle cell disease receiving red cell transfusion therapy will have reduced erythropoietin levels, lower numbers of circulating EPCs and outgrowth cells, better vascular tube formation and reduced oxidant expression than untransfused sickle cell subjects. SPECIFIC AIMS 1. Quantitate the number of circulating EPCs isolated from the peripheral blood of three groups of subjects: Hemoglobin AA (normal), hemoglobin SS without chronic transfusions, and hemoglobin SS on chronic transfusions. 2. Quantitate the number of outgrowth cells derived from EPCs and their rates of senescence in the three groups. 3. Measure vascular tube formation of EPCs from subjects in the three groups. 4. Measure intracellular reactive oxygen species, superoxide levels, and xanthine oxidase activity in the EPCs from the three groups.